Orthostatic intolerance is a leading cardiovascular dysfunction in women of reproductive age. The orthostatic adjustment from seated or lying to standing posture involves the integration of central blood volume and the baroreflex response. The adequacy of the baroreflex response is a function of central baroreceptor sensitivity and peripheral vascular resistance. While a number of studies have indicated that estradiol and progesterone impact aspects of the orthostatic response, no studies have related these changes directly to orthostatic tolerance, and the individual effects of estradiol versus progesterone on orthostatic responses in humans have not been isolated. Estrogens and progesterone have opposing influences on blood pressure regulation, with estradiol tending to increase peripheral vasodilation, and with progesterone tending to increase vasoconstriction. Moreover, estradiol has important effects on progesterone receptor function;thus the interaction of these hormones may also impact the orthostatic response in women. Our experimental design permits suppression followed by controlled estradiol and progesterone administration to determine the individual and combined effects of estradiol and progesterone on baroreflex function and peripheral resistance. On the basis of lower body negative pressure tests to presyncope, we will place subjects into high and low orthostatic tolerance groups in order to test the following Specific Aims: 1) to determine the estradiol and progesterone effects on sympathetic outflow, as well as cardiovagal, sympathetic and integrated baroreflex function;2) to determine the estradiol and progesterone effects on peripheral blood flow and the mechanisms for the sex hormone effects on peripheral blood flow. We expect that these studies will not only increase our understanding of the roles estradiol and progesterone play in blood pressure regulation in women, but may also serve as a basis for interventions to improve orthostatic tolerance in women. We also expect that the data in these studies will provide a basis to study the impact of sex hormones on diseases related to compromised peripheral circulation, such as hypertension, Raynaud's phenomenon and diabetes